This invention relates to a downhole tool for use in a drill string to prevent breakage of the drill string by over torquing.
When rotating drill strings in wells, the lower drill string outside diameter may be typically only about 50 to 100 millimeters. If the drill bit becomes stuck in the hole this is not immediately apparent to the operator who will continue applying torque at the drilling derrick. This can lead to breakage of the drill string deep in the hole which requires a long and expensive fishing operation to retrieve.
The present invention provides a downhole tool comprising a generally tubular body for connection in a drill string, the body comprising an upper tubular member and a lower tubular member interconnected to be axially secured and relatively rotatable, and a frangible element securing the upper and lower members together rotationally and adapted to rupture at a predetermined load.
Preferably, the tool further includes signalling means for signalling to an operator that the frangible element has been ruptured.
The signalling means may generate a signal to the operator in the form of any one or more of the following group; change in torque, change in weight, change in pressure, mud pulse.
The tool may comprise a substantially radial passage allowing for the circulation of fluid from an axial bore through the tool to the exterior of the tool when the frangible element has been ruptured.
The effect of this arrangement is that, if excessive torque is applied to the drill string as a result of for example the drill bit sticking, the frangible element ruptures to uncouple the upper and lower members rotationally. The rupture also opens the passage to allow drilling fluid to flow into the annulus bypassing the drill bit. The bypass of drilling fluid alerts the operator to the incident by virtue of the significantly reduced drilling fluid pressure.
The tool may include means for re-establishing drilling fluid flow through the drill bit after the frangible element has been ruptured, thereby assisting in removing the drill string by continuing the removal of cuttings from the annulus.
In one arrangement, the lower part of the upper tubular member is of reduced diameter and is encircled by the lower tubular member, said passage being formed by radial bores in the overlapping parts of the upper and lower tubular members. Preferably a plurality of passages are provided, typically three or four passages circumferentially equispaced, each having a frangible element in the form of a shear plug.
The tool may include a valve member moveable from a first position permitting fluid flow through the passage to a second position obturating the passage.
The valve member is typically a cylindrical sleeve mounted in the bore of the upper tubular member for sliding movement. The cylindrical sleeve may be secured in its first position by a shear pin, or by a spring-loaded detent or a friction element. The second position of the sleeve may conveniently be defined by abutment surfaces on the sleeve and tubular member.
In one embodiment, the cylindrical sleeve is movable from. its first position to its second position by a ball passed down the drill string bore seating in the sleeve. In a particularly preferred form, the sleeve has axially spaced apertures, upper apertures being aligned with the passages in the first position and lower apertures communicating with the bore of the lower tubular member in the second position, and the ball is an extrudable ball.
The invention encompasses other forms of valve member and operation; or example the valve member could be actuated to its second position by a wireline operation or by signalling, such as mud pulse signalling, to a control circuit within the tool.
The passage may be formed in both the upper tubular member and the lower tubular member, whereby communication between the axial bore of the tool and the exterior of the tool occurs by aligning the respective upper and lower members appropriately.